Time Zone vs. UTC Offset
An offset tells you the difference now. A time zone also helps you work out what happens on another date.
If Amsterdam is UTC+1 today, does that mean it’s always an hour ahead of UTC? No. When the clocks change, its offset changes too. That’s the difference between an offset and a time zone.
A number for one moment
UTC is a time standard used around the world. An offset tells you how far a local clock is ahead of or behind it at a particular time. UTC+1 means one hour ahead; UTC−5 means five hours behind.
That number is handy when you want to describe the clock right now. On its own, though, it doesn’t tell you what will happen when the clocks change.
A place has rules, too
A named time zone such as Europe/Amsterdam gives software a way to look up the local time rules for a particular date. Amsterdam and New York, for example, don’t change their clocks on the same dates, so the difference between them isn’t always the same.
Planning a call next month? Pick the date and both cities in the Time Zone Converter. If several people are joining, the World Clock can show their local times together.
Numerdo uses the time-zone data supplied by your browser. You can read about the database behind named zones at IANA.